The example above configures a framebuffer for two monitors, each using resolutions up to 1680x1050 pixels, put side-by-side dynamically, after you started X. The values are calculated like this:

Virtual <width of the two monitors, e.g. 1680+1680=3360> <maxium height to used by the montiors, e.g. 1050>

Only the part which the monitors show is used: You can start and use X with one monitor as ususal but later add a new monitor and use xrandr with --leftof or --rightof to place the new monitor left or right of your primary monitor.

Preconfigured side-by-side (LeftOf/RightOf) setup thru xorg.conf

If you have done the above, you can also preconfigure the two monitors to be side-by-side right from the X startup. With this, you do not even need the "Virtual " line because then, the X server already knows before starting that it has to allocate a virtual framebuffer which fits both Monitors side-by-side.

You need to make the following changes for this to work (simplest way):

Change the Monitor identifiers in your xorg.conf to the identifiers which "xrandr" reports and add a LeftOf/RightOf to one of the Monitor sections. Example: